DE60100077T2 - A process for the production of poly (urethane-urea) / addition polymer composite particles - Google Patents

Abstract

A process for making poly(urethane-urea)/addition polymer composite particles which avoids the need to use a highly viscous solution of a prepolymer for the poly(urethane-urea). The process comprises dissolving diol and diisocyanate in addition polymerisable monomers and allowing them to co-react but for only long enough to form a precursor for the prepolymer which is of a lower molecular weight than the prepolymer so that the precursor forms a solution of much lower viscosity. This lower viscosity solution is then dispersed in water to give droplets in which the co-reaction continues and completes the formation of the prepolymer whilst water diffuses into the droplets and causes chain extension to create the poly(urethane-urea) particles. The addition polymerisable monomers in the dispersed droplets spontaneously diffuse into the poly(urethane-urea) particles where they are subjected to a conventional free radical addition polymerisation process whereupon composite the poly(urethane-urea)/addition polymer particles are formed. The composite particles can be obtained as stable aqueous dispersions optionally containing less than 3wt% organic solvent and over 40wt% of the composite particles.

Description

This invention relates to a process for preparing poly(urethane-urea)/addition polymer composite particles that does not require handling of viscous materials, and to a process for preparing optionally solvent-free aqueous dispersions of the composite particles. The composite particles have outer regions containing poly(urethane-urea), optionally with some ionic carboxylate groups, and inner regions containing addition polymer. Commercial polyurethanes additionally contain important urea units, and therefore the term "poly(urethane-urea)" is used in this specification to reflect this fact. Addition polymers are polymers (including copolymers) derived from the radical addition polymerization of ethylenically unsaturated relatively hydrophobic monomers such as acrylic (including methacrylic) esters, vinyl esters (usually vinyl carboxylates) or styrene. The invention also relates to aqueous paint compositions as well as adhesive formulations containing the composite particles. Today, environmental considerations increasingly require minimization or preferably total avoidance of organic solvent in aqueous dispersions of particles of this type. Organic solvent-free dispersions would be useful in adhesive formulations as well as paint compositions for wood, concrete, plaster, fiberglass and plastic surfaces of the type found in buildings and metal surfaces of the type found in motor vehicles.

US Patent Specification US 4 644 030 (published in 1987) describes a technique for preparing poly(urethane-urea)/addition polymer composite particles in which a high molecular weight polyurethane prepolymer is first prepared by reacting diol and diisocyanate in addition polymerizable hydrophobic monomers which act as a hydrophobic volatile organic solvent, whereupon the diol and diisocyanate react exothermically to form a viscous solution of a high molecular weight prepolymer in the hydrophobic monomer. After formation of the prepolymer is complete, the solution of the prepolymer in the hydrophobic monomer is dispersed in water as fine droplets. The prepolymer in the droplets undergoes chain extension to form poly(urethane-urea), and the addition polymerizable monomers undergo addition polymerization to produce addition polymer, and the two combine to form poly(urethane-urea) composite particles.

There are two problems with the technique of US 4,644,030. The first is that the intrinsic viscosity of the prepolymer in the monomer is high, so that when dispersing in water one has difficulty in shearing the solution into fine droplets, or this stage of the process is limited to being carried out at high temperatures and/or low concentrations of prepolymer. Ideally the viscosity of the prepolymer/monomer solution should be well below 1 Pa·s at room temperature (say 18ºC). The second problem is that the reaction between the diol and the diisocyanate is exothermic, so there is a risk that premature addition polymerization of the addition polymerizable monomers can be initiated at a local "hot spot", especially if the solution has to be kept hot to keep its viscosity low.

A widely commercially used alternative process to that according to US 4 644 030 relies on the use of solutions of the prepolymer in hot, highly effective, non-polymerizable organic solvents, such solvents necessarily causing unpleasant properties. A typical commercial process using high-efficiency solvents begins with a reaction of aliphatic diisocyanates (although these are more expensive than their aromatic analogues) with dimethylolpropionic acid (which is also expensive) to produce the polyurethane prepolymer dissolved in the organic solvent. The solvent must be aprotic to be inert to the isocyanate and it must also be miscible with water, which in practice leads to the selection of N-methylpyrrolidone, although this is a skin and eye irritant. In addition, the process uses triethylamine as a neutralizing base, although this is pungent and malodorous, and it also uses hydrazine as a chain extender, although this is a carcinogen. N-methylpyrrolidone

The use of all this substance requires specialist handling techniques and thus the process cannot be carried out in the conventional equipment used in the production of polymer latices by addition polymerization of ethylenically unsaturated monomers. A typical process currently in use can be briefly described as follows:

Aliphatic diisocyanate monomers dissolved in N-methylpyrrolidone are reacted in an organic solvent with dimethylolpropionic acid and usually other diols to form a low viscosity solution of polyurethane prepolymer having carboxylic acid groups. An approximate reaction scheme is shown in Fig. 1 of the Figures, where R represents a polymethylene chain (e.g. ethylene) and wherein all reactions involving diols other than dimethylolpropionic acid have been omitted for simplicity.

The carboxylic acid groups are next neutralized with triethylamine to provide the ionic carboxylate groups needed to produce a stable dispersion in water.

The neutralized solution is added to water with vigorous stirring, whereby the prepolymer particles are stably dispersed in the water/N-methylpyrrolidone mixture.

Next, the polyurethane prepolymer is chain extended in the presence of water by adding a diamine (e.g., hydrazine) to the aqueous solution, thereby generating the urea units. An approximate reaction scheme is shown in Fig. 2 of the Figures. Urea units typically improve the resistance to organic solvents and also increase the hardness of the poly(urethane-urea).

The product of the chain extension reaction is an aqueous dispersion of solvent-swollen poly(urethane urea) particles in water containing objectionable residual organic solvent. Thus, the low viscosity solution of the prepolymer in the highly effective organic solvent is obtained only at the expense of an aqueous dispersion contaminated with the organic solvent.

In a final step of the process, an acrylic hydrophobic monomer is introduced into the aqueous dispersion of the swollen poly(urethane-urea) particles under vigorous stirring, where it diffuses into the swollen particles. The diffused monomer becomes one of free radicals (which also diffuse into the swollen particles) induced addition polymerization to produce an acrylic addition polymer. The acrylic polymer is hydrophobic while the poly(urethane-urea) is hydrophilic due to its ionic carboxylate and urea groups. This difference in hydrophobic/hydrophilic character causes the polyurethane-urea) to spontaneously encapsulate the acrylic polymer particles, forming a composite particle having a core/shell structure and which is stably dispersed in water, albeit water contaminated with usually at least 3% by weight and frequently up to 10% by weight of an objectionable and environmentally undesirable residual organic liquid.

In order to form stable aqueous dispersions, existing commercial particles would have to have a large number of ionic carboxylate groups carried by their poly(urethane-urea) polymer. Typically, this large number is equivalent to a nominal acid number of about 30 mg KOH/g polyurethane-urea content of the composite particles. By "nominal acid number" is meant the acid number that the particles would have if the ionic carboxylate groups were converted to carboxylic acid groups. Such high numbers of ionic carboxylate groups are undesirable for two reasons. First, they negatively alter the nature of the compositions in which the particles are formulated and, in particular, they can impart excessive water sensitivity. Secondly, the large number of ionic carboxylate groups interact with the water and organic solvent in an aqueous dispersion, resulting in swelling of the particles and a consequent increase in the viscosity of the aqueous dispersion. For this reason, it is not possible to obtain useful aqueous dispersions of such particles containing more than at best 40% by weight of the highly carboxylated particles, and usually the particle contents are only about 30% by weight.

The process uses aliphatic diisocyanates, although most of their aromatic analogues are much cheaper. The aromatic diisocyanates are rarely used because they react with water about ten times faster than their more expensive aliphatic analogues. It is found that the rate of reaction of water with the aliphatic compounds is slow enough to be tolerable, whereas the cheaper aromatic diisocyanates react so vigorously that too much competition with the diamine chain extension is obtained and also create exothermic conditions leading to foaming (due to the evolution of carbon dioxide) which is difficult to control in a commercial process. The problems resulting from the high rate of reaction of aromatic isocyanates with water are exacerbated by the fact that the reaction is catalyzed by the presence of triethylamine. Accordingly, commercial practice rarely uses aromatic diisocyanates.

The current commercial process also requires the use of expensive diols such as dimethylol alkanoic acids. The reason for this is that an acid diol is the only commercially viable method for introducing carboxylic acid groups into the polyurethane prepolymer. However, there is a problem in that sterically unhindered carboxylic acid groups compete too vigorously for the isocyanate groups. The carboxylic acid group must therefore be sterically hindered and the only commercially viable acid diols which also provide such sterically hindered carboxylic acid groups are the expensive dimethylolpropionic acid and its even more expensive butanoic acid analogue.

N-methylpyrrolidone is chosen as the solvent because it is the least unpleasant of the small group of commercially acceptable aprotic water-miscible organic solvents capable of dissolving the prepolymer and providing a solution having a viscosity low enough to be handleable and dispersible into fine droplets. Even solutions in N-methylpyrrolidone require special handling techniques and cannot be used in a conventional commercial plant for the production of latexes by addition polymerization of ethylenic monomers.

Triethylamine is used as a neutralizing agent for the carboxylic acid because it is the best of the commercially available volatile bases that are also non-reactive with isocyanate, yet strong enough to resist displacement from the salts they form with the ionic carboxylate groups.

Hydrazine is chosen (although it is a carcinogen) because it reacts with the isocyanate more rapidly than water and does not displace the triethylamine from its carboxylates. It can also be used to increase the hardness of the poly(urethane-urea) if necessary.

Despite their cost and the use of toxic and unpleasant materials in their manufacture, poly(urethane-urea)/acrylic core/shell composite particles are required for high quality coating compositions for wood, metal, concrete, fiberglass and certain plastics, as well as in high performance adhesive compositions. They provide dried coating compositions with highly valued resistance to abrasion, scratching and organic solvents, combined with well balanced flexibility and hardness, and they provide dried adhesive compositions with well balanced adhesion and peel strength.

It is an object of this invention to avoid the highly viscous solutions of polyurethane prepolymers, or, alternatively, to avoid the need to use hot solutions of the prepolymer in high potency organic solvents or very dilute solutions of prepolymer in large volumes of solvents. Avoidance of these measures makes it possible for the process to be carried out (if desired) in conventional equipment used for the conventional preparation of polymer latices by addition polymerization of ethylenic monomers. An alternative object is to provide a new process for the preparation of poly(urethane-urea)/addition polymer composite particles which avoids the need for diamine chain extension or the need to use dimethylolpropionic acid in methylpyrrolidone or triethylamine and which allows the preparation of aqueous dispersions containing more than 40% by weight of the particles and less than 3% (preferably 0% by weight) of organic solvent.

Another alternative object of this invention is to provide poly(urethane-urea)/addition polymer composite particles capable of providing aqueous paint compositions or adhesive formulations comparable in performance to currently available compositions or formulations, but without the need for the particles to carry numerous ionic carboxyl groups, preferably none at all. All in all, this means that the particles should have a low nominal acid number of less than 30 mg KOH/g of the poly(urethane-urea) content of the composite particles, preferably they should have no appreciable nominal acid number at all, say less than 5 mg KOH/g. It is also an alternative object to avoid aqueous dispersions contaminated with significant amounts (e.g., greater than 3% by weight) of residual organic solvent.

Accordingly, the present invention provides a process for producing poly(urethane-urea)/addition polymer composite particles which may optionally carry some ionic carboxylate groups, the process comprising:

a) dissolving diol and diisocyanate in an addition-polymerizable hydrophobic monomer to form a hydrophobic solution in which the diol and diisocyanate begin to react with each other,

b) dispersing the hydrophobic solution in water containing a surfactant so that a dispersion of droplets of the hydrophobic solution is formed in the water,

c) forming poly(urethane-urea) by chain extension of a prepolymer formed by the reaction of the diol with the diisocyanate,

d) subjecting the dispersed droplets of the hydrophobic solution to an addition polymerization in which the addition-polymerizable monomers polymerize and form addition polymer domains around which poly(urethane-urea) spontaneously assembles, the process also comprising

e) dispersing the hydrophobic solution in water before the reaction of diol and diisocyanate is complete, thereby forming only a precursor for the prepolymer prior to dispersion in the hydrophobic monomer, said precursor having a molecular weight lower than that of the prepolymer,

f) continuing the reaction of the diol with the diisocyanate in the dispersed droplets of the hydrophobic solution to convert the precursor into the prepolymer, and

g) allowing water to spontaneously diffuse into the dispersed droplets, whereupon chain extension occurs to produce poly(urethane-urea),

thereby obtaining a stable, aqueous dispersion of the poly(urethane urea)/addition polymer particles having a nominal acid number of less than 30 mg KOH per g of the poly(urethane urea) content of the composite particles.

Dispersing the hydrophobic solution in water prior to completion of the reaction of the diol with the diisocyanate ensures that the viscosity of the hydrophobic solution immediately prior to dispersion in water remains low enough to be easily handled and convertible into fine droplets even at room temperatures and high concentrations of the precursors. It was found that the reaction of the diol with the diisocyanate in the dispersed droplets continues with high efficiency despite the presence of water, thereby completing the formation of the prepolymer in the droplets, which remain easily handled because they are stably dispersed in the water. Surprisingly, it appears that the presence of ethylenic unsaturation can help to stabilize the aqueous dispersion of the hydrophobic solution droplets.

Preferably, the dispersion of the hydrophobic solution in the water is carried out before the weight average molecular weight of the precursor has reached at most 80% (preferably 65%) of that of the prepolymer. Limiting the molecular weight of the precursor is necessary in order to obtain hydrophobic solutions which, immediately before dispersion in water, have viscosities (when measured at room temperature, say 18°C) of less than 1 Pa·s and preferably of less than 0.7 Pa·s. The lower viscosity of the solution of the precursor in the hydrophobic solvent means that it is sufficiently easy to store and pump and to disperse in water, enabling the process to be readily adapted to conventional commercial plants used to produce polymer latexes by the addition polymerization of acrylic, vinyl and similar monomers.

By using the process of the invention, it is possible to obtain a stable aqueous dispersion of the poly(urethane urea) particles having a nominal acid number of less than 30 mg KOH/g of the poly(urethane urea) content of the composite particles, and preferably less than 5 mg KOH/g. Nominal acid numbers of substantially zero are readily achievable since it has been discovered that when particles are prepared by the process of the invention, there is no need for carboxyl groups derived from acid diols to enable the composite particles to form stable aqueous dispersions. The low level or absence of carboxylate groups in combination with very low levels or absence of organic solvents enables the aqueous dispersions to contain more than 40% by weight of particles based on the total weight of the dispersion without creating an excessively viscous aqueous dispersion. For commercial purposes, the dispersion typically contains between 45 and 50% by weight of the composite particles.

The process is able to process aromatic diisocyanates and avoid the use of expensive acid diols or unpleasant materials such as N-methylpyrrolidone, triethylamine and hydrazine. Preferably, the diol is added first to the hydrophobic solvent, followed by the diisocyanate. The addition is carried out with stirring and preferably at ambient temperature to form the hydrophobic solution containing diol and diisocyanate. The diol and diisocyanate immediately begin to react with each other and start to form the precursor. The incorporation of short chain water-soluble reactants (particularly simple diols) into the precursor inhibits the loss of such reactants into the water phase when the hydrophobic solution is subsequently dispersed in water. It is preferred not to use elevated temperatures and catalysts such as tin alkanoates at this early stage in order to minimize the risk of an excessive degree of reaction which would produce a solution which is too viscous. The hydrophobic solution is next dispersed in water containing an ionic and/or non-ionic surfactant and this is preferably done at 18 to 25°C and before the viscosity of the solution has reached 0.7 to 1 Pa·s. The dispersion should be carried out under high shear conditions and optionally using ultrasonic vibrations to produce the stable dispersion of droplets having a preferred mean diameter of about 40 to 700 nm and more preferably 40 to 250 nm.

Anionic surfactants introduce carboxylate groups and assist in the formation of a stable dispersion of fine particle size droplets, while nonionic surfactants assist in the maintenance of a stable dispersion under high shear conditions in the presence of metal ions. Preferred anionic surfactants include the sodium salts of dioctyl sulfosuccinate, dodecylbenzenesulfonate, dodecyl sulfate, and nonylphenol ethoxylate sulfonates. Preferred nonionic surfactants include alkyl ethoxylates, alkylphenol ethoxylates, and block copolymers of ethylene oxide with propylene oxide, and reactive surfactants such as methyl poly(ethylene glycol) methacrylate, alkenyl succinic anhydride condensates, or alkoxylated allyl ethers. Reactive surfactants can bond with the addition polymer domains to provide dispersions of composite particles that exhibit improved colloidal stability under high shear conditions and in the presence of organic solvents. Possible cationic Surfactants include cetyltrimethylammonium bromide or amine derivatives of alkyl or alkylphenol ethoxylates or of block copolymers of ethylene and propylene oxides.

Once the droplets are dispersed in water, the reaction between diol and diisocyanate can be safely accelerated. The droplets can therefore be exposed to temperatures of 30 to 80°C (30 to 50°C), optionally in the presence of preferably 0.005 to 0.2% by weight of a tin catalyst, to create conditions that promote reaction of the diol with the diisocyanate to accelerate completion of the formation of the polyurethane prepolymer. Suitable tin catalysts include dibutyltin dilaurate and dibutyltin acetate. The tin catalysts also catalyze chain extension by water. Alternative catalysts include tetrabutyl titanate and tin, titanium, zirconium and bismuth carboxylates such as bismuth neodecanoate. It is preferred to add the catalysts to the hydrophobic solution immediately before its dispersion in the water/surfactant mixture in order to facilitate the dispersion of the catalyst within the dispersed droplets.

The isocyanate groups are able to react with both the hydroxy groups in the diol and with the small amounts of water that slowly diffuse into the dispersed droplets. This gives the composite particles a character that is somewhat different from that of conventional poly(urethane-urea)/addition polymer composite particles, although the difference has not yet been precisely defined. Primary hydroxy groups react more rapidly with isocyanate than water, so the reaction is favored to complete the formation of the polyurethane prepolymer. Completion of prepolymer formation is also favored by the slow rate at which the water diffuses into the droplets. Nevertheless, some reaction with water does occur, and some Chain-extending urea units are formed at this stage of the process, improving the distribution of urea units, resulting in better resistance to organic solvents as well as the hardness of coatings obtained from the particles.

The isocyanate groups are preferably used in a numerical ratio of 1.0 to 3.0 to the hydroxy groups of the diols and any polyols. This means that the polyurethane prepolymer tends to be formed with residual isocyanate groups. As the diol hydroxy groups are consumed by the reaction, the reaction with water is subject to a decreasing competitive reaction, allowing the water to increasingly chain extend the prepolymer by forming urea units, particularly with residual isocyanate groups. Such chain extension causes crosslinking and/or branching. It is believed that the chain extension follows the following reaction scheme: Urea Unit

Chain extension with water is actually preferred over chain extension with a diamine, since the diamine both overcatalyzes the competing water/isocyanate reaction and also promotes the flocculation of poly(urethane-urea). Thus, the poly(urethane-urea) contains urea units, such as those suggested above. but need not contain diamine residues since no diamine needs to be used in the chain extension. Chain extension of the polyurethane prepolymer provides a poly(urethane-urea) having a weight average molecular weight (Mw) of at least 10,000 (preferably 20,000 to 500,000).

It has been found that the reaction between aromatic diisocyanates and water is attenuated by conducting it in the droplets of the hydrophobic solution and that in the absence of the catalytic action of a tertiary amine such as triethylamine, it is expected that the slow rate at which water diffuses into the droplets is sufficient to make the process of the invention commercially viable even when carried out using aromatic diisocyanates.

The diols used in the process according to the invention can be simple diols such as neopentyl glycol or 1,4-butanediol or polymeric glycols such as poly(ethylene glycol), poly(propylene glycol) or poly(tetramethylene glycol) with number average molecular weights (Mn) of 200 to 7,000 and above, but preferably of 500 to 3,000.

The molecular weight of the diol is one of the factors that affects the hardness of a poly(urethane-urea) particle. In general, higher molecular weights produce softer particles, such as are preferred for use in adhesives. In contrast, lower molecular weight diols produce harder particles. Thus, diols containing from 85 to 95 weight percent polymeric diols of over 500 Mn are preferred when the particles are desired for use in adhesives, while 5 to 35 (preferably 10 to 20) weight percent simple diols may be preferred when the particles are intended for use in paint compositions for wood, metal or plastics.

The diols can also be polyesters formed by reacting diols as described above with dicarboxylic acids such as adipic acid, isophthalic acid, terephthalic acid or fumaric acid or acid anhydrides such as phthalic, maleic or succinic anhydrides. Preferably, the polyesters have number average molecular weights, Mn, of 200 to 10,000, but Mn is particularly preferably between 500 and 3,000. The diols can optionally be used in combination with polyols having three or more hydroxy groups in order to increase the molecular weight of the prepolymer.

The diisocyanates used in the process are preferably aromatic diisocyanates, optionally used in combinations with mono- or poly-isocyanates, where a poly-isocyanate is an isocyanate containing at least three isocyanate groups. Suitable aromatic diisocyanates include methylene di-p-phenyl di-isocyanate and 2,4-toluene di-isocyanate. Aliphatic isocyanates are more expensive, but can be used to achieve increased resistance to ultraviolet light.

The process can be modified by varying the selection of isocyanates to adjust the properties of the poly(urethane-urea)/addition polymer composite particle. For example, the molecular weight of the polyurethane prepolymer can be increased by replacing a small amount (say up to 5 mole %) of the diisocyanate with a polyisocyanate containing three or more isocyanate groups. Alternatively, the molecular weight can be lowered by replacing a small amount (say up to 5 mole %) of the diisocyanate with a mono-isocyanate. The molecular weight can also be increased by replacing a small amount of the diol with (say up to 5 mole %) of a polyol containing three or more hydroxy groups.

Other possible modifications include the introduction of acid moieties into the poly(urethane-urea) to improve its adhesion to substrates. This can be achieved by replacing a small amount (say up to 5 mol%) of the diol with dimethylolpropionic acid or with a diol that is a polyester.

The addition polymerization of the ethylenically unsaturated hydrophobic monomer can be carried out using any of the conventional free radical techniques used for the polymerization of acrylate or vinyl ester monomers. Examples of suitable initiators which provide free radicals by thermal addition include azo compounds such as 2,2-azobis(isobutyronitrile), persulfates such as ammonium persulfate and peroxides such as hydrogen peroxide, lauryl peroxide or tert-butyl peroxy-(2-ethylhexanoate). A suitable redox system for generating free radicals is the iron-ascorbic acid-hydrogen peroxide system and this can be used at temperatures of 50°C or below. Such low temperatures are preferred to minimize the risk of reaction overheating. The addition polymerization of the hydrophobic monomer is preferably carried out simultaneously with the reactions involving the isocyanate, except that chain extension may continue for some time after the completion of the addition polymerization.

The preferred ethylenically unsaturated monomers are relatively hydrophobic monomers such as C₁ to C₈ alkyl ethers of acrylic or methacrylic acid, vinyl esters such as vinyl acetate or the so-called vinyl "versatate" (vinyl "versatate" is the vinyl ester of the so-called "versat" acid, which is a mixture of aliphatic monocarboxylic acids, each of which has an average of 9, 10 or 11 atoms, and is commercially available from the Shell Chemical Company of Carrington, England) or styrene, all of which are ambient conditions. Acid moieties can be introduced by using liquid monomers such as acrylic or methacrylic acid, or solid acid anhydrides such as maleic or succinic anhydrides, provided that the anhydrides are soluble in the liquid monomers. An addition copolymer can be given a suitable glass transition temperature (Tg) by choosing a combination of monomers some of which, when homopolymerized, would give polymers having a high Tg while the others would give polymers having a low Tg. For example, by choosing a monomer whose homopolymer has a Tg of less than 310K and one whose homopolymer has a Tg of above 330K, it is possible to create a copolymer having a Tg of 270 to 340K and which is therefore pleasantly soft at ambient temperatures. Alternatively, by increasing the amount of monomers whose homopolymer has a high Tg, one can obtain a copolymer useful for higher temperature applications such as those used in industrial coating or bonding processes. Water immiscibility is an important property of the hydrophobic monomers used as solvents. Higher poly(urethane-urea) to addition polymer ratios can be achieved by replacing part of the addition polymerizable monomer with an inert hydrophobic organic solvent, but at the expense of having to remove the inert solvent before the aqueous dispersions of the composite particles can be used in fully environmentally friendly coating compositions.

A possible modification of the addition polymerization process leads to a more direct bond between addition polymer domains and the poly(urethane-urea). The modification involves replacing a small amount (say up to 5 mol%) of the addition polymerizable hydrophobic monomer with a hydroxyalkyl (meth)acrylate monomer. The Hydroxyalkyl monomer copolymerizes into the addition polymer particles, providing hydroxy groups which, due to their more hydrophilic nature, tend to migrate toward the surface of a composite particle. The hydroxy groups are therefore available for formation of urethane bonds with the polyurethane prepolymer and serve to create a direct positive bond between the outer poly(urethane-urea) regions of the composite particle and its inner addition polymer regions. Such bonding can improve the abrasion and scratch resistance of dried compositions containing composite particles.

Another possible modification of an addition polymerization process results in the formation of smaller domains of addition polymer. The smaller domains have the advantage that when the aqueous dispersions of the composite particles are used to form coatings, the resulting dried coatings are much clearer, i.e., they are less translucent and more transparent. The modification involves replacing a small amount (say, from 0.1 to 2.5 mole %) of the addition polymerizable hydrophobic monomer with a crosslinkable monomer. The most convenient crosslinkable monomers contain multiple ethylenic unsaturation. Preferred doubly unsaturated monomers include diethylene glycol dimethacrylate, butanediol diacrylate, hexanediol diacrylate, allyl methacrylate, and diallyl phthalate. Preferred triunsaturated monomers include trimethylolpropane triacrylate, glycerol triacrylate, and pentaerythritol triallyl ether.

It is not fully understood why cross-linkable monomers have such a beneficial effect on clarity, but it is possible that cross-linking reduces the mobility of the growing chains of the addition polymer and/or traps some of the poly(urethane-urea). Both effects could promote the growth of the domains It is therefore possible that a composite particle may contain a plurality of domains of addition polymer at least partially surrounded by poly(urethane-urea), with the further possibility that some of the poly(urethane-urea) may be present within a domain. Such a multi-domain structure is an alternative to the simpler core-shell structure in which the composite particle comprises a single domain or "core" of addition polymer around which is disposed an at least partial "shell" of poly(urethane-urea).

The poly(urethane-urea)/addition polymer composite particles obtained by the process of the invention preferably have particle sizes in the nanosize range of 40 to 700 nm, and in particular they can be nanosize particles having particle sizes of 40 to 250 nm. The particles can be obtained as aqueous dispersions in water which do not contain any appreciable amounts of residual organic solvent (at least less than 3% by weight and preferably substantially 0% by weight). The dispersions are therefore particularly useful in applications where environmental protection is important. The aqueous dispersions can contain over 40% by weight of particles (based on the total weight of the dispersion) and particle contents in the range of 45 to 50% by weight are preferred.

Aqueous dispersions of composite particles prepared by the process of the invention can be added to aqueous paint compositions and adhesive formulations to improve the abrasion, scratch and solvent resistance of the dried coatings and the adhesive and solvent resistant properties of dried adhesive formulations. They are particularly useful in paint compositions for architectural surfaces such as walls, ceilings, door and window frames and in particular for floors. More generally, they are useful for coating compositions for wood, metal, concrete, fiberglass and plastic, provided that the plastic surface has sufficient polar characteristics. They are useful for coating compositions for motor vehicles and metal cans. The coating compositions may also contain conventional additives such as pigments, dyes, extenders, thickeners and biocides. In applications where less aggressive organic coalescing solvents can be tolerated, the quality of the dry films formed can be improved by adding conventional coalescing solvents such as 2,2,4'-trimethyl-1,3-pentanediol monoisobutyrate or 1-methoxy-2-hydroxypropane. Preferably, the amount of coalescing solvent should be from 1 to 1.15% by weight of the coating compositions.

The invention is further illustrated by the following examples.

EXAMPLE 1 Preparation of hard poly(urethane-urea)/addition polymer composite particles for a wood coating composition:

The amounts of the different reactants used are shown in Table 1.

A mixture of ethylenically unsaturated addition polymerizable hydrophobic monomers, which were methyl methacrylate, butyl acrylate and styrene, was added to a round bottom flask at room temperature. Into the monomer mixture were stirred neopentyl glycol (a simple diol) and a polyester diol, which was a polyester of neopentyl glycol and butanediol with adipic acid with a number average molecular weight of about 1000 to form a homogeneous solution of diols in a hydrophobic monomer mixture. A mixture of 4,4'- and 2,4'-methylene-di-phenyl-di-isocyanate was then stirred into the hydrophobic monomer mixture, where it also dissolved. The diol began to react with the diisocyanate and a solution of a precursor for the later polyurethane prepolymer began to form in the hydrophobic monomer mixture. The viscosity of the flask contents was kept below 1 Pa·s, measured at 18ºC.

After only about 5 minutes and well before the completion of the diol and diisocyanate reaction, demineralized water containing an anionic surfactant, sodium dioctyl sulfosuccinate, was slowly added to the contents of the flask under conditions of high shear mixing produced by a Silverson homogenizer. Silverson homogenizers are supplied by Silverson Machine Limited of Chesham, England. A dispersion of fine droplets of the hydrophobic monomer mixture solution was obtained which was then subjected to further dispersion using a Branson ultrasonic homogenizer for 4 minutes. Branson homogenizers are supplied by Branson Ultrasonics of Danbury, Connecticut, USA. The final dispersion obtained contained nano-sized droplets of a hydrophobic monomer mixture solution with a "z"-average diameter of 200 nm as determined by photocorrelation spectroscopy.

The aqueous dispersion of nano-sized droplets was transferred to a 2-liter container where it was stirred and maintained under a nitrogen atmosphere while the reaction of the diol with the diisocyanate continued, converting the precursor into the prepolymer. Water slowly diffused into the nano-sized droplets, initiating slow chain extension and leading to the formation of poly(urethane-urea) particles.

The temperature of the dispersion was raised to 80°C and tert-butyl peroxy-(2-ethylhexanoate) was added as a radical initiator to initiate an addition copolymerization of the hydrophobic monomers, which proceeded for 3 hours at 80°C. The addition polymerization was then completed by adding another small amount of the initiator together with some ascorbionic acid and heating was continued for a further 45 minutes.

After standing overnight, it was found that over 95% of the isocyanate groups had reacted to form poly(urethane-urea) arranged as an outer shell around an inner core of methyl methacrylate/butyl acrylate/styrene addition copolymer. The nanosized core/shell composite particles thus prepared had a "z" average particle size of 188 nm and comprised 50 wt% addition copolymer core and 50 wt% poly(urethane-urea) shell. The particles were obtained in an aqueous dispersion containing 41.8 wt% composite particles and less than 0.1 wt% residual organic solvent. The particles were found to have a nominal acid number of 11.5 mg KOH/g of the poly(urethane urea) content of the composite particles.

TABLE 1 Reactants used in Example 1

Reactant / weight g used

Methyl methacrylate 65.0

Butyl acrylate 41.2

Styrene 110.6

Neopentyl glycol 19.4

Neopentyl glycol/butanediol/adipate (mol.wt. 1000) 93.6

Methylene diphenyl diisocyanate 105.2

Demineralized water 533.9

Anionic surfactant: Sodium dioctyl sulfosuccinate 14.0

Tertiarybutylperoxy-(2-ethylhexanoate)

First encore 4.3

Second encore 2.1

Ascorbic acid as a 5 wt.% solution in water 10.7

Total 1000.0

In a modification of the above procedure, 0.01 g of dibutyltin dilaurate was added as a catalyst for the chain extension reaction, and as a result, the poly(urethane-urea) formation and chain extension were essentially completed after 2 hours.

A sample of the aqueous dispersion of composite particles prepared according to this example was mixed with 10% by weight of a coalescing cosolvent which was 1-methoxy-2-hydroxypropane. The mixture was coated onto a glass surface using a 200 µm block coater, dried at 120°C for 30 minutes, allowed to cool to ambient temperature (18°C) and then allowed to stand at that temperature for 24 hours. The dried coating was then subjected to the Erichsen Pendulum Hardness Test which directly correlates hardness to the time required to completely dampen the oscillation of the pendulum. The dried coating required a time to dampen the pendulum which was comparable to the time required for a A corresponding coating was required which had been manufactured in accordance with the state of the art.

EXAMPLE 2 Use of alternative diols:

Example 1 was repeated but using the reactants shown in Table 2 to demonstrate the suitability of alternative diols.

The resulting aqueous dispersion contained composite particles with a "z"-average particle size of 209 nm and was stable after 5 minutes of shear at 10,000/s applied by an ICI cone and plate viscometer. The dispersion could be drawn out to a thickness of 200 µm, whereupon it dried to give a clear film of good flexibility, having an Erichsen hardness of 80 seconds after 7 days.

TABLE 2 Reactants used in Example 2

Reactant / weight g used

Methyl methacrylate 66.4

Butyl acrylate 42.1

Styrene 112.9

¹ "Pripol" 2033 71.8

Neopentyl glycol/²DMPA/adipate (molecular weight 1000) 62.1

Methylene diphenyl diisocyanate 69.7

Demineralized water 543.2

Anionic surfactant: sodium dioctyl sulfosuccinate 14.3

Tertiarybutylperoxy-(2-ethylhexanoate)

First encore 4.4

Second encore 2.2

Ascorbic acid as a 5 wt.% solution in water 10.9

10.7

Total 1000.0

¹ "Pripol" 2033 is a dimeric diol obtained from a mixture of branched chain C36 fatty acids and is supplied by Uniquema BV, Gouda, the Netherlands.

² DMPA is dimethylolpropionic acid in a polyester derived from 55.5 wt% neopentyl glycol, 11.6 wt% DMPA and 52.2 wt% adipic acid.

EXAMPLE 3 Use of polypropoxylate as diol:

Example 2 was repeated except that the reactants were those shown in Table 3, and which particularly show a change to a non-ionic surfactant and a redox initiator system. The first make-up addition of initiator was made 30 minutes after the first initiator addition, and the second and third make-up additions were made in the same manner after further intervals of 30 minutes each.

An aqueous dispersion of composite particles was obtained that was very similar to that of Example 2, except that the particles had a "z" average particle size of 217 nm. The dispersion gave a similar hard dried film. The dispersion showed promise as a size for glass fibers.

TABLE 3 Reactants used in Example 3

Reactant / weight g used

Butyl acrylate 58.7

Styrene 81.7

Polypropoxylate (molecular weight 2000) 56.2

Neopentyl glycol 11.7

Methylene diphenyl diisocyanate 112.4

Demineralized water 323.7

Non-ionic surfactant: "Synperonic" F88 14.0

543.2

14.3

Redox Initiator:

Oxidizing agents

Hydrogen peroxide (10% solution) 16.5

Demineralized water 18.1

Reducing agent

Ascorbic acid (5% solution) 18.8

Demineralized water 18.1

Refresher initiator (1)

Hydrogen peroxide (10% solution) 3.2

Ascorbic acid (5% solution) 3.5

Refresher initiator (2) 3.2

Hydrogen peroxide (10% solution) 3.5

Ascorbic acid (5% solution)

Refresher initiator (3)

Hydrogen peroxide (10% solution) 3.2

Ascorbic acid (5% solution) 3.5

Total 750.0

The drawings accompanying this description illustrate possible reaction schemes used in existing processes.

Fig. 1 shows a reaction scheme for the formation of a common polyurethane prepolymer.

Figure 2 shows a reaction scheme for the chain extension of the prepolymer shown in Figure 1 with hydrazine.

Claims (14)

1. A process for producing poly(urethane-urea)/addition polymer composite particles which may optionally carry some ionic carboxylate groups, the process comprising a) dissolving diol and diisocyanate in an addition polymerizable hydrophobic monomer to form a hydrophobic solution in which the diol and diisocyanate begin to react with each other, b) dispersing the hydrophobic solution in water containing a surfactant so that a dispersion of droplets of the hydrophobic solution is formed in the water, c) forming poly(urethane-urea) by chain extension of a prepolymer formed by the reaction of the diol with the diisocyanate, d) subjecting the dispersed droplets of the hydrophobic solution to an addition polymerization in which the addition-polymerizable monomers polymerize and form addition polymer domains around which poly(urethane-urea) spontaneously assembles, The procedure also includes e) dispersing the hydrophobic solution in water before the reaction of diol and diisocyanate is complete, whereby only a precursor for the prepolymer is formed prior to dispersion in the hydrophobic monomer, said precursor having a molecular weight lower than that of the prepolymer, f) continuing the reaction of the diol with the diisocyanate in the dispersed droplets of the hydrophobic solution to convert the precursor into the prepolymer, and g) Allowing water to diffuse spontaneously into the dispersed droplets, resulting in chain extension producing polyurethane urea), thereby obtaining a stable, aqueous dispersion of the poly(urethane urea)/addition polymer particles having a nominal acid number of less than 30 mg KOH per g of the poly(urethane urea) content of the composite particles. 2. The process of claim 1, wherein the hydrophobic solution is dispersed in the water before the weight average molecular weight of the solution has risen above 80% of the weight average molecular weight of the prepolymer. 3. A process according to claim 1 or claim 2, wherein the hydrophobic solution is dispersed in the water before the viscosity of the solution, measured at 18°C, exceeds 1 Pa·s. 4. A process according to any one of the preceding claims, wherein the hydrophobic monomer solution is dispersed in the water under shear conditions sufficiently high to produce nano-sized droplets of the monomer solution having a number average diameter of 40 to 400 nm. 5. A process according to any one of the preceding claims, wherein the diol is dissolved in the hydrophobic solvent before the diisocyanate. 6. A process according to any one of the preceding claims, wherein the diisocyanate is an aromatic diisocyanate. 7. A process according to any one of the preceding claims, wherein up to 5 mole percent of the addition polymerizable monomer is a hydroxyalkyl (meth)acrylate monomer. 8. A process according to any one of the preceding claims, wherein up to 2.5 mole percent of the addition polymerizable monomer is a crosslinkable monomer. 9. An aqueous dispersion, the dispersion comprising composite particles prepared according to any one of claims 1 to 8. 10. An aqueous dispersion according to claim 9, wherein the dispersion contains less than 3% by weight of organic solvent. 11. An aqueous dispersion according to claim 9 or claim 10, wherein the dispersion contains more than 40% by weight of the composite particles. 12. An aqueous coating composition, the composition containing composite particles prepared by a process according to any one of claims 1 to 8. 13. An aqueous adhesive formulation, the formulation containing composite particles prepared by a process according to any of claims 1 to 8. 14. A substrate made of wood, metal, concrete, fiberglass or plastics when coated with a film formed at least partially from composite particles produced by a process according to any one of claims 1 to 8.